In a digital camera which obtains an image signal using an imaging element, such as a CCD, a COMS, or the like, gamma correction is applied to the image signal captured by the imaging element to thereby establish linearity on an output device (mainly, a display panel). More specifically, taking into account that a display apparatus, such as a display panel, generally has a gamma (γ) value of approximately 2, gamma correction using the reciprocal of the gamma value (which is also referred to as inverse gamma correction, but is simply referred to as “gamma correction” in this specification) is applied to an input signal to thereby linearize (establish linearity of) characteristics of an output relative to an input.
Gamma correction is generally implemented by storing gamma correction values previously computed for an input signal as a gamma correction table in a memory, such as a ROM, and referring to an address in the ROM in accordance with a value of the input signal, to thereby output, as an output signal, the gamma correction values (having been gamma corrected) stored in the corresponding address. For example, 10-bit input signal values and their corresponding 8-bit output values are prestored in the ROM, and, when an input signal value is “153”, the corresponding output value of “130” is provided as an output value. Because gamma correction values are generally smaller than 1, a curve depicting outputs relative to inputs (gamma correction curve) has a convex shape which is upwardly protruded.
On the other hand, because utilization of fixed gamma correction values might introduce a phenomenon in which shadowed images are completely blacked out (rendered as a featureless block of pixels of a single color), especially in dark areas, depending on the imaged subject, a technology of modifying gamma correction values for low-brightness areas has also been suggested.
Japanese Patent Laid-Open Publication No. 2004-23605, for example, describes that detection means for detecting a signal of a high-brightness subject is provided for adjusting exposure time, to thereby prevent highlighting in an image of the high-brightness subject from being blacked out. When images are captured with such an adjusted exposure time, because the shadow of a dark subject is blacked out, the characteristics of gamma correction means are also changed to compensate for a blacked out shadow by increasing the brightness level of dark areas.
Further, Japanese Patent Laid-Open Publication No. 2003-87604 describes that, taking into account the fact that when an image signal from an CCD is adjusted in gain by an AGC, noise in a dark area (a dark noise) becomes noticeable as the gain increases, the characteristics of the gamma correction means are changed so as to reduce the brightness level of the dark areas in response to increases of the gain of the AGC.
Further, Japanese Patent No. 3551655 teaches that each histogram for R, Q and B signals is created to calculate a gamma correction coefficient in accordance with a degree of deterioration in gradation for each of the R, G, and B signals.
In the above-described conventional technologies, the blacked out shadow and the dark noise are suppressed by locally modifying gamma correction values for dark areas in the gamma correction means. However, users often wish to perform photography in a flexible and more accurate manner in consideration of the subject to be photographed. Specifically, when a user photographs a subject under automatic exposure (AE) control, for example, a blue sky which the users wishes to be clearly represented might be saturated in a captured image of the subject, or when the user photographs a flower using a macro focus mode, red might be saturated in a captured image of the flower. On the other hand, there are various user demands including, for example, a request for enhancing a contrast of a captured image whose overall brightness is low. In order to respond to such a user demand, it is necessary that a dynamic range of an imaging element, such as a CCD, or the like, be changed in accordance with the brightness of a subject to be photographed.
When only the dynamic range is simply shifted, however, a relative position of a reference amount of incident light (for example, a 18% gray level corresponding to human skin), which is used as a target for a correct exposure under automatic exposure control, in the dynamic range varies in response to the shifting of the dynamic range, which results in that a captured image will be unnatural as a whole. Even though the gamma correction values in the low-brightness area are upwardly or downwardly modified using the above-described conventional technologies, the above-described problem would not be resolved because the 18% gray level is not affected by such a modification to the gamma correction values in low-brightness areas.